Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

Metallogenic Belts Formed During Middle
Paleozoic Rifting of North American Craton
Margin or in Low-Temperature Brines
Along Craton Margin
Mystic Metallogenic Belt of SEDEX Bedded Barite
and Southeast Missouri Pb-Zn Deposits
(Belt MY) West-Central Alaska
The Mystic metallogenic belt of SEDEX massive bedded barite and Southeast Missouri Pb-Zn deposits occurs in West-
Central Alaska (fig. 17; tables 3,4) (Nokleberg and others, 1997b, 1998). The belt is hosted in the Mystic and Nixon passive
continental margin terranes (Nokleberg and others, 1994c, 1997~). The significant deposits are at Gagaryah and Reef Ridge. The
belt also contains younger a younger Besshi massive sulfide(?) deposit at Shellebarger Pass. In addition, very high Cu background
values (350 to 450 ppm Cu) occur in Late Triassic basalt, and several small syngenetic Cyprus-type chalcopyrite deposits occur
within interstices of pillow structures and in aquagene tuff of the Mystic terrane in the McGrath quadrangle (T.K. Bundtzen,
written commun., 1992).
Bedded Bariteand Southeast Missouri Pb-Zn Deposit
A sedimentary-exhalative (SEDEX) bedded barite deposit is hosted Gagaryah at in Late Devonian (Frasnian) shales and
clastic rock host barite mineralization in the Lime Hills D-4 Quadrangle (Bundtzen and Gilbert, 1991). The deposit consists of
nodular, laminated, composite, and massive, light gray barite in Frasnian (early Late Devonian) shale, limestone, and minor chert
of Mystic Terrane. The deposit extends along strike for 640 meters, has an average thickness of 20 meters, and an estimated down-
dip extension of 300 meters. The deposit contains slightly elevated levels of Av, V, Sr (in celestite), but no lead or zinc. Sulfide
isotopic analyses of +20 and +24 determined from nodular and massive barite respectively. The deposit contains 2.3 million
tonnes grading 51% barite. The barite is interpreted as deposited syngenetically into host shale basin with barite rapidly
precipitating from low temperature hydrothermal fluids distal from exhalative vents. Barite nodules and spheroids are also
commonly encountered in either Devonian or Mississippian strata at other localities in the Mystic terrane to the northeast.
A Southeast Missouri Pb-Zn deposit at Reef Ridge consists of stringers of brown sphalerite and minor galena in
hydrothermal breccia in carbonate rocks of the Silurian and Devonian Whirlwind Creek Formation in the Nixon Fork terrane
(Harold Noyes, written cornmun., 1984). The deposit extends along sh-ike for 2,000 m and ranges up to 15 m thick. The sulfides
pinch and swell along strike. The deposit is the best known of ten similar nearby occurrences, and contains bout 181,000 tonnes of
15% combined Zn and Pb.
Shellebarger Pass Besshl Massive Sulfide(?) Deposit
The younger Shellebarger Pass Besshi massive sulfide(?) deposit (Reed and Eberlein, 1972; Bundtzen and Gilbert, 1983)
consists of a very fine grained mixture of mainly pyrite and marcasite and lesser sphalerite, chalcopyrite, galena, and pyrrhotite in
a gangue of siderite, calcite, quartz, and dolomite. The sulfides and gangue occur in massive, lenticular sulfide bodies, as
replacements of carbonate-rich beds, and as fracture fillings, mainly in chert and siltstone. The host rocks are a Triassic and (or)
Jurassic age sequence of chert, dolomite, siltstone, shale, volcanic graywacke, conglomerate, aquagene tuff, and overlain by an
upper sequence of pillow basalt, agglomerate, and breccia. At least six individual sulfide bodies are known. The main sulfide
bodies may be proximal to basaltic flow fronts. The highest chalcopyrite concentrations occur in the basal parts of bodies. Minor
sphalerite occurs in or near the hanging wall. Extensive hydrothermal alteration occurs in the footwall, but is to absent in hanging
wall. The basalt displays high background Cu values of 250 to 300 glt. The deposit contains an estimated several hundred
thousand tonnes of unknown grade. Individual samples contain up to 5% Cu and average about 2% Cu and 1% Zn.
Origin of and Tectonic Controls for
Mystic Metallogenic Belt
The Mystic metallogenic belt is hosted in the Mystic and Nixon passive continental margin terranes which consist of a
complexly deformed but partly coherent, long-lived stratigraphic succession, including Devonian through Pensylvanian carbonate
and clastic sedimentary rock, Permian flysch and chert, and Triassic(?) pillow basalt (Nokleberg and others, 1994c, 1997~):
Recent studies report early to middle Paleozoic fauna in these terranes that are typical of taxa that occur in similar age units in the
Kolyrna region in the Russian Northeast and suggest that these three terranes were rifted from the Siberian continent (North Asian
Craton Margin) (Blodgett and Brease, 1997; Blodgett, 1998; Fryda and Blodgett, 1998; Dumoulin and others, 1998, 1999;
Blodgett and Boucot, 1999). The Mississippian and older parts of these terranes and a stratigraphy that is similar to the North
Asian Craton Margin (NSV). Accordingly, these Mississippian and older parts of these terranes and their Mississippian and older
lode SEDEX bedded barite deposits and metallogenic belts are herein interpreted as being derived from rifting of the North Asian
Craton Margin (NSV) (Nokleberg and others, 2000). Coeval metallogenic belts with similar origin deposits residing in the Russian
Northeast include the Urultun and Sudar Rivers, Selennyakh River, and Sette-Daban, and Yarkhodon belts (table 3). The tectonic